Acid gases are an undesirable impurity in materials such as, for example, petroleum hydrocarbons because several of these gases such as COS and H2S are a sulfur source and therefore a potential atmospheric pollutant. COS and H2S also act as an undesirable contaminant of industrial processes such as, for example, by poisoning of polymerization catalysts when the contaminant is present in petroleum-derived polymerizable olefins such as propylene. The acid gases may be introduced into such processes as a contaminant, initially present in the feedstock or they may be formed in the treating process as a result of the molecular sieve-catalyzed reaction of carbon dioxide with hydrogen sulfide or other sulfur compounds. For example, acid gases can be found in natural gas streams, and besides being a pollutant, acid gases such as COS, H2S, CO2, CS2, SO2, HCl, HF and HBr can be corrosive to natural gas pipelines, pipeline equipment, and other chemical processing apparatus.
Depending upon the process and the required purity of the product, COS levels in the starting material may be required to be reduced to below 1 part per million by weight (ppmw) and sometimes to levels below 100 part per billion by weight (ppbw). Concentration of COS in the range of a few ppmw cannot be separated efficiently from a petroleum feedstock such as propylene by fractional distillation because the boiling point of COS differs from propylene by only 3.4° C.
Khelghatian U.S. Pat. No. 3,315,003 teaches a process for removing COS from a hydrocarbon by first contacting the hydrocarbon with a liquid such as monoethanolamine which scrubs the hydrocarbon to remove acid gases such as H2S and CO2 and part of the COS. The hydrocarbon is then distilled. After several subsequent distillations, the liquid bottom product is treated with a soda-lime to remove any remaining COS.
However, separation of COS by processes which involve distillation, in addition, are extremely costly due to the cost of energy to vaporize virtually all of the liquid. It is, therefore, desirable to provide other means for the removal of COS impurities from organic liquids.
It has also been proposed to remove COS from hydrocarbons by catalytic hydrolysis to form H2S, for example, using alumina as a catalyst. Frevel et al U.S. Pat. No. 3,265,757 teaches the hydrolysis of COS contained in a liquid hydrocarbon by contacting a mixture of the liquid hydrocarbon and water, at a temperature of from 20 to 50° C., with a high surface area alkaline, active alumina containing from 0.15 to 3 wt. % of sodium or potassium. The patentees state that the hydrolysis reaction will not commence, however, if the alumina is bone dry. They suggest either moistening the alumina catalyst with ion-free water prior to the reaction or passing a mixture of ion-free water and the liquid hydrocarbon through the catalyst bed until a sufficient amount of water has built up on the alumina to permit the hydrolysis reaction to proceed. However, while this process does remove COS (by converting it to H2S), it does not remove sulfur from the hydrocarbon, but merely changes the form of the sulfur compound which still must be subsequently removed from the hydrocarbon by another process step.
In a later patent dealing with the same type of reaction, Polleck et al U.S. Pat. No. 4,491,516 teach that the reaction rate for the hydrolysis of COS with water over alumina may be greatly increased if the ratio of water to COS ranges from 1 to 10 moles of water per mole of COS, preferably 1.5 to 6 moles of water per mole of COS, or about 30% of saturation of the hydrocarbon, whichever upper limit provides the lesser amount of water.
Brownell et al U.S. Pat. No. 4,455,446 teaches the removal of COS from propylene by hydrolysis over a catalyst comprising platinum sulfide on alumina. The patentees state that the hydrolysis reaction may be carried out in either the gaseous or liquid phase with a temperature of 35° to 65° C. used for the liquid phase. An amount of water at least double the stoichiometric amount of the COS to be hydrolyzed must also be present.
Harris et al U.S. Pat. No. 4,391,677 describe a process for desulfurizing a butene-1 rich stock containing sulfurous impurities such as H2S, COS, and CH3SH. The process comprises passing the feed stream through a desulfurization zone maintained under desulfurization conditions and containing a charge of at least one desulfurization medium capable of adsorbing, absorbing, or converting H2S, COS, and CH3SH to high boiling sulfurous compounds. The thus-treated feed stream, now essentially free from H2S, COS, and CH3SH, is then passed to a distillation zone, and recovered as a bottom product as a butene-2 rich stream containing high boiling sulfurous compounds. The desulfurization zone comprises a bed of activated alumina followed by a bed of zinc oxide. The activated alumina is said to hydrolyze COS in the presence of 20 to 1000 ppm of water to H2S and partially to remove H2S and methyl mercaptan. The zinc oxide is said to remove all the H2S and methyl mercaptan not removed by the alumina bed.
COS has also been removed from liquid hydrocarbons by adsorption on a zeolite adsorbent. Collins U.S. Pat. No. 3,654,144 discloses removing COS by adsorbing it on a particular modified zeolite A adsorbent comprising an alkali metal cation form of zeolite A which has been ion-exchanged with alkaline earth metal cations, preferably calcium cations, to the extent of from 20 to about 100 equivalent percent.
Innes U.S. Pat. No. 4,098,684 describes the removal of COS and other sulfur compounds by passing them through a dual bed of zeolites comprising, respectively, a 13× molecular sieve, and a zeolite A sieve having a pore size of 4 Angstroms. The commercially available 13× zeolite is said to remove any H2S and mercaptans present. The capacity for COS adsorption by the 13× sieve is said to be small. The 13× zeolite is described as a three dimensional network with mutually connected intracrystalline voids accessible through pore openings which will admit molecules with critical dimensions up to 10 Angstroms and having the general chemical formula: 0.83.+−.0.05 Na2O/1.00 Al2O3/2.48.+−.0.038 SiO2. The molecular sieve beds may be regenerated by passing a hot, substantially nonadsorbable, purge gas through the beds at a temperature of about 177° to 316° C.
While zeolite materials have thus been used as adsorbing agents to remove sulfurous compounds such as COS from liquids hydrocarbons, it has been found that zeolite, with its cage structure, has a low adsorption rate at ambient temperature and is, therefore, not practical for treating liquids at such temperatures.
It would, therefore, be highly desirable to provide a process for the removal of acid gases including sulfurous impurities such as COS from liquids or gases, preferably in the absence of water, using an alumina adsorbent having high adsorption characteristics yet capable of being regenerated without substantial loss of adsorption capability. Removal of acid gases other than COS from liquids or gases to minimal ppm levels using an alumina absorbent would also be desirable.
U.S. Pat. No. 4,835,338 to Liu provides an improved process for the removal of carbonyl sulfide from a liquid hydrocarbon by adsorption on an adsorption media comprising an activated alumina adsorbent followed by regeneration of the activated alumina after the adsorption capacity has been reached. The activated alumina adsorbent is pretreated with a compound selected from the class consisting of one or more alkali metal compounds, one or more alkaline earth metal compounds, or mixtures of any two or more of such compounds; then used to adsorb carbonyl sulfide from a hydrocarbon; and then regenerated by passing a gas through the adsorbent. Useful activated alumina is disclosed as a commercial product having a particle size range of from ¼inch to 100 mesh (150 microns). In practice, the alumina particles are formed by agglomerating 5 micron alumina powders into the larger particles suitable for the adsorption process.